Systems and methods to test and/or recover magnetic sensors with esd or other damage

ABSTRACT

A system according to one embodiment includes a power supply for charging a lead of a magnetic sensor to a voltage; an interface for operatively coupling the power supply to the lead of the magnetic sensor; a relay for selectively coupling the lead of the magnetic sensor to ground for causing a discharge event, wherein the discharge event reverses a magnetic orientation of a pinned layer of the magnetic sensor; and a shorting resistor between the relay and ground.

BACKGROUND

The present invention relates to data storage testing and recoverysystems, and more particularly, this invention relates to systems andmethods to test and/or recover sensors with Electrostatic Discharge(ESD) and other magnetic damage.

ESD damage to Giant Magnetoresistive (GMR) sensors is a major source ofyield loss for magnetic GMR read sensors used in tape and hard diskstorage drives as well as other applications. The damage mechanism whichhas one of the lowest current/voltage thresholds for damage is thepinned layer reversal. A pinned layer may be present in a magneticsensor to act as a reference to the free layer and/or to stabilize otherlayers. Pinned layer reversal may occur at current/voltage levels abouthalf those required for permanent damage. When diodes are connected inparallel with the GMR sensor, the sensors are damaged in the same manneras without diodes, except the damage thresholds are increased by factorsof 5 to 10 or more.

The standard method of recovering a GMR sensor with a pinned layerreversal is to apply a bias current to the sensor of in the appropriatebias direction to favor normal pinned layer orientations and with a highenough magnitude and of an appropriate pulse width (time duration) tocause the reversal to the proper orientation while not causing damage tothe sensor.

Pulse generators have been used to achieve the appropriate results. Theproblem with prior approaches have been the need for expensive pulsegenerators. Another problem with programmable pulse generators is thatshort time duration pulse generators have limited current levels,usually 10 to 100 mA for a 10 ns pulse width, and even less for shorterpulse durations. When diodes protection is applied to the sensors, thecurrents required to cause a pinned layer reversal for a pulse width of<10 ns can be of the order of 1 A or more. Standard, inexpensive,programmable pulse generators are not designed for generating currentshigh enough to recover the pinned layer reversal using short timepulses, and require expensive special equipment. One approach used writedrivers in a hard disk drive (HDD) to pulse the head for recovery ofheads without diode protection. Since write drivers typically aresupplied by about 5 to 10 V maximum, recovering a diode protected sensorwill probably not be possible, since the voltages required can be 50 Vor more. Furthermore, in the case of tape drives, where the number ofsensors is 36 or more in extant drives, cost of the circuitry to switchthe sensors between the pulse and the normal operation is significant,since each drive would require the relay switches for each sensor.

Another issue with recovery is the desire to know whether the sensor isrecovered. To measure the recovery, one typically measures the sensorresponse to an applied external magnetic field, such as an electromagnetor by reading data written on magnetic media. These methods areimpractical and expensive. Reading data from magnetic media requiresmounting the head onto a magnetic tester to read the media, which istime consuming. Applying an external magnetic field on the sensor willnot work with head which has a magnetically actuated head. For example,tape heads contain magnetic actuators which could be damaged by magnetictorques if placed in a strong homogeneous magnetic field.

BRIEF SUMMARY

A system according to one embodiment includes a power supply forcharging a lead of a magnetic sensor to a voltage; an interface foroperatively coupling the power supply to the lead of the magneticsensor; a relay for selectively coupling the lead of the magnetic sensorto ground for causing a discharge event, wherein the discharge eventreverses a magnetic orientation of a pinned layer of the magneticsensor; and a shorting resistor between the relay and ground.

A system for testing a magnetic sensor according to one embodimentincludes a discharge circuit to cause a discharge event on a magneticsensor; a bias generation circuit to apply at least one first biascurrent to the sensor and at least one second bias current to thesensor, the second bias current being different than the first biascurrent; a resistance determination circuit to determine a resistance ofthe magnetic sensor at each of the applied bias currents; and a damagedetermination circuit to determine whether the magnetic sensor isdamaged and/or was fixed by a discharge event based on the resistances.

A method according to one embodiment includes measuring resistances of amagnetic sensor at multiple bias currents; determining that the magneticsensor is damaged based on the measured resistances; selecting a biasvoltage sufficient to cause a discharge event that repairs the damagedmagnetic sensor to a proper magnetic state thereof; and applying thebias voltage to the magnetic sensor; and coupling the lead of themagnetic sensor to ground after applying the bias voltage for causingthe discharge event, wherein the discharge event fixes the damagedmagnetic sensor.

Any of these embodiments may be implemented in a system such as a tapedrive system, which may include a magnetic head, a drive mechanism forpassing a magnetic medium (e.g., recording tape) over the magnetic head,and a controller electrically coupled to the magnetic head.

Other aspects and embodiments of the present invention will becomeapparent from the following detailed description, which, when taken inconjunction with the drawings, illustrate by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic diagram of a pinned layer recovery systemaccording to one embodiment.

FIG. 2 depicts current through R_(s) versus cable voltage, V_(cc)according to one embodiment.

FIG. 3 is a plot of the change in quasi magnetic amplitude versus biascurrent.

FIG. 4 a is a plot of R_(pn-scaled) versus cable voltage for a cabledsensor using a CDE technique according to one embodiment.

FIG. 4 b is a plot of R_(pn-scaled) versus bias current for a cabledsensor using the CDE technique according to one embodiment.

FIG. 5 a is a plot of a Quasi Amplitude Transfer curve (hysteresis loop)for a sensor with diode protection.

FIG. 5 b is a plot of ΔR_(pn-scaled) for the sensor of FIG. 5 afollowing CDE events of different current/voltage levels.

FIG. 6 a is a plot of R_(pn) versus current for Cycle0 and Cycle6.

FIG. 6 b is a plot of R_(pnscaled) versus current for Cycle0 and Cycle6.

FIG. 7 is a plot of resistance versus bias current for a magneticallygood (R5) and a magnetically damaged (R4) GMR sensor.

FIG. 8 is a plot of the mean-squared deviation ofRloop=Σ|Rup(I)-Rdown(I))| versus GMR Sensor Track number.

FIG. 9 a is plot of change in R_(pnIscale) versus voltage for a CDEevent of servos and readers with diode protection.

FIG. 9 b is plot of change in R_(pnIscale) versus current for a CDEevent of servos and readers with diode protection.

FIG. 10 is a schematic diagram of a simplified tape drive systemaccording to one embodiment.

FIG. 11 illustrates a side view of a flat-lapped, bi-directional,two-module magnetic tape head according to one embodiment.

DETAILED DESCRIPTION

The following description is made for the purpose of illustrating thegeneral principles of the present invention and is not meant to limitthe inventive concepts claimed herein. Further, particular featuresdescribed herein can be used in combination with other describedfeatures in each of the various possible combinations and permutations.

Unless otherwise specifically defined herein, all terms are to be giventheir broadest possible interpretation, including meanings implied fromthe specification as well as meanings understood by those skilled in theart and/or as defined in dictionaries, treatises, etc.

It must also be noted that, as used in the specification and theappended claims, the singular forms “a,” “an” and “the” include pluralreferents unless otherwise specified.

The following description discloses several preferred embodiments ofsystems, as well as operation and/or component parts thereof.Embodiments of the present invention use a discharge event, referred toillustratively as a cable discharge event (CDE) (noting that an actualcable may or may not be involved), to apply a current pulse to amagnetic sensor to cause the recovery of pinned layer magneticorientation from a reversed state and/or removal of multiple domains orto anneal the sensor. The CDE in one approach involves charging thesensor with a power supply and subsequently discharging the sensorthrough a resistor by closing a relay (e.g., switch, transistor, etc.).The system may preferably be calibrated for current versus voltage andmeasurements may preferably be made to determine the charge voltage usedto recover the sensor's pinned layer orientation and/or remove themultiple magnetic domains or to anneal the sensor. Simple, inexpensivepower supplies, which may be employed in some embodiments, can, ifdesired, easily supply currents of an Ampere or more by choosing theappropriate charging voltage and relying on the capacitive charge storedin the cable or added capacitor. Such power supplies can also becontrolled to a high precision, for example supplying voltages accurateto 1% or better.

To determine whether the sensor is recovered, the tester can measure theresistance (R_(p)=R(I_(bias))) of the sensor with a current of +I_(bias)and (R_(n)=R(−I_(bias))) at a current of −I_(bias). An appropriatechoice of values for would be currents whose magnitude is in theoperating range of the sensor. I_(bias) applies an internal magneticfield to the free layer (FL) of the sensor, causing the resistance tochange due to the GMR affect. A current of −I_(bias) will rotate themagnetic field in the sensor FL in the opposite direction as with theI_(bias). Since the current also heats the sensor via Joule heating,which can cause significant resistance changes, measuring the resistanceat both +I_(bias) and −I_(bias) and taking the difference will allow oneto discern the GMR effect over and above the Joule heating.

Detection of damage to and/or recovery of a GMR sensor may be made usinga variety of methods, as disclosed herein. In one approach, thedetection of damage and/or recovery uses the parametersR_(pn)(I_(bias)), and/or R_(pnI)(I_(bias)), and/orR_(pnscaled)(I_(bias)), and/or R_(pnIscaled)(I_(bias)), which are givenby Equations 1a, 1b, 1c and 1d:

R _(pn)(I _(bias))=[R(+I _(bias))−R(−I _(bias))],  1a.

R _(pnI)(I _(bias))=R _(pn)(I _(bias))/I _(bias),  1b.

and

R _(pnscaled)(I _(bias))=200%*R _(pn)(I _(bias))/[R(+I _(bias))+R(−I_(bias))]].  1c.

and

R _(pnscaled)(I _(bias))=200%*R _(pnI)(I _(bias))/[I _(bias) *[R(+I_(bias))+R(−I _(bias))]].  1d.

Though Equations 1a-d represent several useful forms of analyzing thedifference R_(p)−R_(n), they are not the only forms, and one coulddetermine other choices which utilize the same or similar concepts. Suchother choices should be considered in the same class of equations. Oneimportant concept may be to take an average <f> of one of the parameters(f(I_(bias)) over a range of currents:

<f>≡Σf(I _(bias))_({Ibias})  2.

Since R_(pn)(I_(bias)) should be linear in I_(bias), <R_(pn)> and<R_(pnI)> are good choices of functions to utilize Equation 2.

In one general embodiment, a system includes a power supply for charginga lead of a magnetic sensor to a voltage; an interface for operativelycoupling the power supply to the lead of the magnetic sensor; a relayfor selectively coupling the lead of the magnetic sensor to ground forcausing a discharge event, wherein the discharge event reverses amagnetic orientation of a pinned layer of the magnetic sensor; and ashorting resistor between the relay and ground.

In one general embodiment, a system for testing a magnetic sensorincludes a discharge circuit to cause a discharge event on a magneticsensor; a bias generation circuit to apply at least one first biascurrent to the sensor and at least one second bias current to thesensor, the second bias current being different than the first biascurrent; a resistance determination circuit to determine a resistance ofthe magnetic sensor at each of the applied bias currents; and a damagedetermination circuit to determine whether the magnetic sensor isdamaged and/or was fixed by a discharge event based on the resistances.

In another general embodiment, a method includes measuring resistancesof a magnetic sensor at multiple bias currents; determining that themagnetic sensor is damaged based on the measured resistances; selectinga bias voltage sufficient to cause a discharge event that repairs thedamaged magnetic sensor to a proper magnetic state thereof; and applyingthe bias voltage to the magnetic sensor; and coupling the lead of themagnetic sensor to ground after applying the bias voltage for causingthe discharge event, wherein the discharge event fixes the damagedmagnetic sensor.

Description of CDE

FIG. 1 is a schematic diagram of a pinned layer recovery system 10according to one embodiment. As will become apparent when reading thedescription of the FIG., such system may include one or more of thecircuits listed herein, such as a discharge circuit to cause a dischargeevent on a magnetic sensor; a bias generation circuit to apply at leastone first bias current to the sensor and at least one second biascurrent to the sensor, the second bias current being different than thefirst bias current; a resistance determination circuit to determine aresistance of the magnetic sensor at each of the applied bias currents;and/or a damage determination circuit to determine whether the magneticsensor is damaged and/or was fixed by a discharge event based on theresistances. Moreover, one or more of these circuits may be part of adifferent system, e.g., computer, processor, etc., in communication withthe system 10 of FIG. 1.

As shown in FIG. 1, a GMR sensor 12 is mounted on a cable 14, where thecable 14 may or may not be present, and may or may not be part of thesystem 10. The negative lead 16 of the sensor connects to Relay 1 18.The b side of Relay 1 18 connects to the negative side of the dataacquisition module DAC 20 (DAC−), which is discussed in more detailbelow. The positive lead 22 of the sensor 12 is coupled to Relay 2 24.The coupling of the positive lead 22 to Relay 2 24 and the negative lead16 to Relay 1 18 within system 10 is done via an appropriate interface23, which may include a cable; pads, pins or a socket for couplingdirectly to the sensor; pads, pins or a socket for coupling to a cablethat is coupled to the sensor; etc. The “a” side of Relay 2 24 isconnected to the CDE power supply 26 and the “b” side connects to thepositive side of the DAC 20 (DAC+). The CDE power supply 26 charges upto a voltage Vcc, which may be applied to a voltage divider of two highimpedance resistors, R_(L) 40 and R_(H) 41. Reasonable values of R_(L)and R_(H) are and 1 MΩ and 100 kΩ respectively, but could be higher orlower. Usually, one would choose R_(H)<<R_(L) so that the charge on thecable (V_(cable)) approaches V_(cc). R_(H) and R_(L) are also bothchosen to be >>R_(s), so V_(cable) approaches zero when the CDE event isover. R_(H) is also chosen large enough so that charging is slow enoughto avoid damaging the sensor when V_(cc) is turned on. This avoidscomplex charging algorithms. R, may have a resistance of less than about500 ohms, but preferably about 10 ohms or lower.

In the discussion that follows, it is assumed that current flow from thenegative lead 16 of the GMR sensor 12 to the positive lead 22 of the GMRsensor 12 results in a reversal of the pinned layer magnetization to theundesired orientation, while current flow from the positive lead 22 tothe negative lead 16 of the GMR sensor 12 leads to proper orientation ofthe pinned layer magnetization. In other approaches, the opposite may betrue.

The following process may be used to recover a magnetically damagedsensor, where it is assumed that current flow from the negative lead 16of the GMR sensor 12 to the positive lead 22 of the GMR sensor 12results in a reversal of the pinned layer magnetization to the undesiredorientation, while current flow from the positive lead 22 to thenegative lead 16 of the GMR sensor 12 leads to proper orientation of thepinned layer magnetization. Referring to FIG. 1, Relay 1 18 and Relay 224 are set to the “a” position, and Relay 3 28 is set to open, a-side.The positive lead 22 of the GMR sensor 12 is charged to a negativevoltage by V_(cc). The positive lead 22 of the GMR sensor 12 is thenrapidly connected to system ground 30 through shorting resistor R_(s) 32by switching Relay 3 28 to the b side. Upon the discharge, positivecurrent flows from the positive lead 22 through the negative lead 16 ofthe GMR sensor 12, termed forward bias current flow. If the current ishigh enough, the head will recover. The current passing through theshorting resistor, R_(s), 32 is linear with respect to the cablevoltage, V_(cc), as is seen in FIG. 2. Particularly, FIG. 2 depictscurrent through R_(s) versus cable voltage, V_(cc).

It is desirable to while switching relays, current pulses. To increasethe probability of avoiding unwanted current pulses are avoided, allleads may be connected to signal ground through high resistanceresistors, R_(d) 34, 36, 38, 39 and 43 which may have the same values ordifferent values. An illustrative resistance value for R_(d) of about1-10 MΩ is also a good choice given the values of R_(N) and R_(L) givenabove. Lower values for Rd, such as down to about 50Ω could be used, butthis depends on the complex circuit and the choice for R_(H), R_(L), andR_(s). Since during the charging process, R_(d) 34, R_(d) 39 and R_(d)43 are in parallel, with R_(L) 40, R_(d) 34, 39 and R_(d) 43 arepreferably chosen to be comparable or larger than R_(L) 41 to ensurethat the cable is charged sufficiently, i.e. V_(cable) approachesV_(cc). Note that the cable is charged to a voltage Vcable of:

V _(cable) =V _(cc) *R _(L2) /[R _(L2) +R _(H)],  3a.

where

1/R _(L2)=1/R _(L)40+1/R _(d)34+1/R _(d)39+1/R _(d)43.  3b.

It is believed that shorter pulse widths are better in terms of reducingthe chance of causing permanent damage to the sensor. Accordingly, thepulse width of the discharge event preferably has a duration of lessthan about 45 nanoseconds (ns) and greater than 0 ns, where “about X”means X±10%. In some approaches, the pulse width of the discharge eventis less than about 10 ns, and in further approaches is less than about 5ns, and in yet other approaches less than about 1 ns.

In other approaches, however, it may be desirable to increase the pulsewidth. This may allow use of lower voltages to reverse the pinned layer.In one approach, a ground plane of the cable 14 may be coupled to groundvia a relay 42. Note that the cable ground is coupled to the cable leadsby a capacitance C_(g) 45.

In some approaches, it may be desirable to include one or morecapacitors such as C₁, C₂, and/or C₃ to assist in obtaining the desiredpulse shape. Such capacitors may have a capacitance in the 1 s or 10 sof picofarads or more, and may each have a different capacitance. Forexample, one embodiment includes a capacitor C₁ 44 in the path betweenthe magnetic sensor and ground. This has the effect of increasing thecharge in the pulse caused by the discharge event and reduces thevoltage needed to reset the pinned layer. The capacitor also broadensthe pulse width, so it offers an inexpensive means of adjusting thepulse width. Moreover, capacitor C₂ 48, coupled between another lead ofthe sensor 12 and ground, and in the embodiment shown in parallel withresistor 39, is preferably implemented when the cable ground plane 46 isnot connected to the Vcc ground plane 30 (floating). Capacitor C₃ isparticularly useful in embodiments having a floating cable ground plane.Capacitor C₃ may have a value in the few to tens of picofarads.

It is typical for high speed cards to use ground planes for noisereduction. In some embodiments, however, the system does not include aground plane in the system board. Systems not having a ground plane havebeen found to avoid damage to sensitive elements when the relays areswitched (toggled). Removal/omission of the ground plane avoidsintroduction of an electronic coupling between the sensors and relayswhen switching that, in experiments, using more sensitive GMR elements,surprisingly and unexpectedly damage the more sensitive elements 12.

In addition, an inductive choke of any type, such as a ferrite core, maybe coupled to one or more of the leads 27, 29 from the power supply,thereby creating a high impedance path for unwanted currents such asground loop currents. Preferably, the inductive choke provides aninductance of greater than about 1000 microhenry, but optimally greaterthan 0.5 millihenry. In one approach, the lead(s) may be wrapped arounda ferrite core. Use of an inductive choke is particularly preferred inembodiments having no ground plane, as the ground loops tend to dominatesuch systems. These in turn give broad LRC oscillations. In ourexperiments, with no inductive choke, the LRC oscillations had periodsof the order of 100 to 250 ns, yielding pulses of the order of 50 ns ormore, which results in a very narrow set voltage (current) windowavailable to fix heads.

The sensor and/or cable may include crossed diodes that operativelycouple the two leads of the magnetic sensor by providing a lowresistance shunting path for the bulk of an ESD or CDE current to bypassthe GMR sensor. Though the bulk of the ESD current will pass through thediodes in a high current ESD or CD event, a fraction of the current willpass through the sensor. The values of V_(cable) required to repair acabled GMR sensor with diode protection may increase 50 to 100 timesover that required to repair a non-diode protected GMR sensor, requiringvoltage values of 10 s to 100 s of Volts, which is easily achieved usinginexpensive power supplies. The system described herein works even withthe presence of such diodes, as the voltage is applied to one leg of thesensor. An illustrative scheme includes crossed diodes coupling theleads of the sensor together. The crossed diodes may be formed on thechip comprising the sensor, may be formed on or coupled to a cablecoupled to the sensor leads, etc. U.S. Pat. No. 7,548,405, which isherein incorporated by reference, discloses illustrative ESD protectionschemes employing diodes that may be used in conjunction with thesensors disclosed herein.

The DAC 20, if present, is a mechanism that is used to measure theresistance of the sensor 12 at various bias currents. The DAC 20 mayprovide the bias currents to the sensor in one or both polarities (dualpolarity). A resistance R_(p) is observed when current flows from thepositive sensor lead (connected to DAC+) to the negative sensor lead(connected to DAC−), and R_(n) is observed when current flows in thereverse direction (reverse polarity).

In one approach, the multiple bias currents include at least one currentat a positive polarity and at least one current at a negative polarity.In another approach, the multiple bias currents include several currentsat a positive polarity or several currents at a negative polarity. TheDAC 20 in one approach is a dual polarity DAC that applies the currentsto the magnetic sensor.

A method for determining whether a sensor is damaged and/or whether thesensor has been repaired by a CDE according to one embodiment includesmeasuring resistances of a magnetic sensor at multiple bias currents,e.g., using a DAC capable of supplying currents of both positive andnegative polarity. Again, the bias currents may be all of one polarity,but preferably include currents at both polarities. A determination ismade, e.g., by the DAC, that the magnetic sensor is damaged based on themeasured resistances. A bias voltage sufficient to cause a dischargeevent that fixes the damaged magnetic sensor is selected and applied tothe magnetic sensor. Illustrative methods for selecting the proper biasvoltage will be evident from the following description. The lead of themagnetic sensor to which the voltage was applied is selectively coupledto ground for causing the discharge event, where the discharge eventfixes the damaged magnetic sensor.

Detection of Magnetic Damage and Recovery Using Pos-Neg Test: R_(Pn) orR_(pnscaled) or R_(pnI) or R_(pnIscaled).

As noted above, magnetic sensors may suffer from a variety of magneticdamage mechanisms, including pinned layer reversal, multiple magneticdomains within the anti ferromagnetic (AFM) or the PL or FL of thesensor, or degradation of the sensor due to permanent physical damagesuch as metal layer interdiffusion or electromigration or melting.

The GMR magnetic response of a sensor has been traditionally measuredusing an external magnetic field and measuring the change in resistanceof the sensor versus external field. The quasi amplitude (Amp) isdefined as the change in voltage of the GMR sensor at +H_(field) and−H_(field) using a constant current (I_(bias)) source as in Equation 3:

Amp=V(+H _(field))−V(−H _(field))=I _(bias) *[R(+H _(field))−R(−H_(field))]  4.

FIG. 3 plots the change in quasi magnetic amplitude versus CDE current.The current pulse is achieved by charging the cable to a given voltage,V_(cable), by applying a voltage V_(cc), and then discharging thecharged cable through R_(s). As shown in FIG. 2, I_(pulse) is linearlyproportional to V_(cable) (V_(cc)). Vcc (V_(cable)) is positive forsensors R2 and R4 and negative for sensor R3. Thus, the current pulsethrough the sensor is reverse biased for R2 and R4 and forward biasedfor R3. A pinned layer reversal occurs for R2 and R4 but not for R3. Thechange in amplitude, compared to the initial amplitude (ΔAmp), ismeasured tiller each current pulse (I_(pulse)) and plotted versusI_(pulse). A negative change of greater than −100% indicates a pinnedlayer reversal. For the parts studied, the reversal occurred at currentsof about 120 mA (6.3 V) for reverse bias pulses (R2 and R4). The sensorR3, which was subjected to forward bias pulses, did not undergo pinnedlayer reversal. At about 175 mA (9.3 V), the amplitude began to change,and the magnitude of the amplitude, |Amp| began to increase with currentpulse. This is permanent damage associated with metal diffusion withinthe GMR stack. Total destruction occurred between about 200 and 225 mA(10.6 and 11.9 V). Note that once a pinned layer is reversed, forwardbias currents act upon the new magnetic state in the same manner asreverse bias currents act upon a GMR with a normal GMR magnetic state.That is, a GMR with a reversed pinned layer will flip back to the normalmagnetic state by applying a forward biased CDE current with a currentpulse between about 120 and 175 mA (6.3 and 9.3 V). The result is a GMRwith a normal pinned layer orientation, with stable magnetic layers.

Detection of the magnetic state of a GMR sensor can also be made usingthe Pos-Neg test, which uses the parameters R_(pn) or R_(pnI) orR_(pnscaled) or R_(pnIscaled) given earlier by Equations 1a, 1b, 1c and1d. The change in R_(pn) (ΔR_(pn)) or R_(pnI) (ΔR_(pnI)) or R_(pnscaled)(ΔR_(pnscaled)) or R_(pnIscaled) (ΔR_(pnIscaled)) from their initialvalues following a CDE pulse can be used to determine the state of theGMR sensor, e.g., whether it is still damaged or whether it has beenrepaired.

FIGS. 4 a and 4 b respectively plot R_(pnIscaled) versus cable voltageand bias current using the same data points as shown in FIG. 3 with thequasi amplitude. Vcc is positive for R2 and R4 and negative for R3. Apinned layer reversal occurs for R2 and R4 but not for R3. Comparison ofFIGS. 3, 4 a and 4 b clearly show that R_(pnIscaled) can be reliablyused to both verify that a sensor is damaged via a pinned layerreversal, and to verify that the sensor has recovered after the pulse.

Besides pinned layer reversal, another form of magnetic damage is thepresence of multiple domains within the sensor. Magnetic domains areobserved as open loops or kinks in the sensor response curve(Amp(H_(field))) to an external magnetic field (H_(field)) also known asa transfer curve. FIG. 5 is a plot of Quasi Amplitude versus H_(field)for Cycle0 through Cycle 6. The reader therein had diode protection. Thereader was initially damaged having magnetic domains. The domains showedup both as an open loop and a kink in the hysteresis loop for Cycles 0through 5 in FIG. 5. Both the open loop and the kink represent majordeviations from linearity of Amplitude versus H_(field). In Cycle6, thesensor is normal, as seen by the linear Amplitude versus H_(field). Therecovery of this sensor at Cycle6 will be discussed later.

R_(pnscaled) and R_(pnIscaled) versus bias current can also be used todetect the presence of magnetic domains in sensor. FIG. 6 a plotsR_(pnscaled) and FIG. 6 b plots R_(pnIscaled) versus I_(bias) for Cycles0 and 6 for the same head as shown in FIG. 5 for the Quasi Amplitude.The presence of domains in Cycle 0 shows up in the deviation of bothR_(pnscaled) and R_(pnIscaled) from expected results of linear responseversus bias current and an in the expected magnitude of R_(pnscaled) orR_(pnIscaled). To determine the magnetic state of the GMR sensor, theparts can be fit to a first order polynomial according to Equations 5aand 5b.

R _(pn-Fit) =R _(pno) +M _(pn) *I _(bias),and  5a.

R _(pnIscaled-Fit) =R _(pnISo) +M _(pnIS) *I _(bias).  5b.

FIGS. 6 a and 6 b respectively are plots of R_(pn) and R_(pnIscaled)versus I_(bias). The figures also show linear fits R_(pn-Fit) andR_(pnIscaled-Fit). Table 1 summarizes the parameters used to make thefit. The slope M_(pn) and the intercept R_(pno) are 54% and 36% higherfor Cycle0 than for Cycle6, yielding higher magnitudes for R_(pn) forthe former. The same is true for R_(pnIscaled), where the slope M_(pnS)and the intercept R_(pnSo) are 13% and 88% higher for Cycle0 than forCycle6, yielding higher magnitudes for R former. R_(pnIscaled) for theFinally, the mean square deviation (msd) for both R_(pn) (σR_(pn)) andR_(pnIscaled) (σR_(pnIS)) from their linear fits are substantiallyhigher for Cycle0 than for Cycle6: 4.4 times for R_(pn) and 5.2 timesfor R_(pnIscaled). All of these are a result of the presence of multipledomains in the Cycle0 state compared to the Cycle6 state.

TABLE 1 Parameters for fitting R_(pn) and R_(pnlscaled). R_(pn)R_(pnlscaled) R_(pno) M_(pn) σ_(Rpn) R_(pnlSo) M_(pnlS) σ_(RpnlS) (%)(%/mA) (%) (%/mA) (%/mA²) (%/mA) Cycle0 0.518 −0.556 0.124 −0.333−0.0224 0.217 Cycle6 0.382 −0.360 0.028 −0.177 −0.0199 0.042 Ratio 1.361.54 4.43 1.88 1.13 5.17

One can determine the expected values for σR_(pn) and σR_(pnIS) usingvarious techniques. For example, one could determine their values from alarge population of good parts. Another means can be used in tape headsor other magnetic read devices which have multiple GMR devices within asingle module device which were all processed from the same wafer inadjacent locations. In such a case, one could take the median of thevalues and choose the parts which are within pre-determined range of themedian, such as:

N_(s)*σ_(product),  5c.

where σ_(product) is the mean-squared deviation of a large number ofparts, and N_(s) is a user defined number, such as 1 or 1.2 or 2, or 2.5or 3, which determines how tight one wants to accept the variance of theparts. Any part which is within N_(s)*σ_(product) of the median can beused to determine the expected value of σ_(Rpn) or σ_(Rpnscaled) for thesensor in question.

Detection of Magnetic Damage—Other Approaches

Various other methods may be used to detect damage to magnetic sensorsand/or verify whether the sensor has been repaired by a CDE, any ofwhich may be used by the various embodiments. Several methods areillustrated herein.

In one approach, the resistance values or values derived therefrom arecompared to some comparative data, e.g., similar values derived fromother sensors under similar conditions, data derived from manufacturingprocesses or verification, design parameters, etc. For example, R_(pn)values for a plurality of read sensors may be measured, and any sensorhaving R_(pn), values deviating from the average or median R_(pn) valuesof the group by some predetermined amount can be marked as beingdamaged.

In another approach, a “looping” measurement of resistance is made, suchas shown in FIG. 7. In FIG. 7, the resistance is measured versus biascurrent. The current is stepped from I_(min) of 1 mA to I_(max) of 7 mAin increments of +1 mA, and back down to 1 mA in decrements of −1 mA.The current is then stepped from −I_(min) of −1 mA to −I_(max) of −7 mAin decrements of −1 mA, and back up to 1 mA in increments of +1 mA. Tworesistance values are measured each current (except for ±I_(max)). Anopen loop is evident in the “Damaged” sensor curve (R4). For example,the two resistances at −5 mA are distinctly different. The measuredpoints of the “Good” sensor (R5) are reproducible irrespective of thehistory of the current used to arrive at a given resistance.

To assist in describing and quantifying the curves, one can defineR_(up)(I_(bias)) as the resistance measured at incrementally increasingcurrent values between I_(min) and I_(max) (or −I_(max) and −I_(min))(inclusive) at increments of ΔI and R_(down)(I_(bias)) is measured fromincrementally decreasing current values between I_(max) and I_(min)(inclusive) (or −I_(min) and −I_(max)) at increments of −ΔI Thefollowing equations can be used to further understand the curves:

R_(loop) _(—)_(p)=Σ_({Ibias})|R_(up)(I_(bias))−R_(down)(I_(bias))|,  6a.

and

R_(loop) _(—)_(n)=Σ_({Ibias})|R_(up)(−I_(bias))−R_(down)(−I_(bias))|,  6b.

and

R_(loop)=R_(loop) _(—) _(p)+R_(loop) _(—) _(n)  6c.

and

σ_(RLoop) _(—)_(p)=Σ_({Ibias})[R_(up)(I_(bias))−R_(down)(I_(bias))]²,  6d.

and

σ_(RLoop) _(—)_(n)=Σ_({Ibias})[R_(up)(−I_(bias))−R_(down)(−I_(bias))]²,  6e.

and

σ_(RLoop)=σ_(RLoop) _(—) _(p)+σ_(RLoop) _(—) _(n)  6f

R_(loop) _(—) _(p), R_(loop) _(—) _(n), and R_(loop) are a crudeintegral of the areas between the up and the down loops for the positiveand negative currents respectively. σ_(RLoop) _(—) _(p) and σ_(RLoop)_(—) _(n), and σ_(RLoop) is the sum of the two. Are another measure ofthe deviation of the parts, and is 2*the mean-squared difference ofR_(up) and R_(down) from the average of R_(up) and R_(down). R_(loop)_(—) _(p) and/or R_(loop) _(—) _(n) and/or σ_(RLoop) _(—) _(p) and/orσ_(RLoop) _(—) _(n) and/or σ_(RLoop) should be below some predeterminedvalue if the sensor is undamaged. The predetermined value can come froman average or a median of a large number of good parts with a rangedetermined by the standard deviation of the values using Equation 5c.FIG. 8 is a plot of σ_(RLoop) using all measured currents for {I_(bias)}(1, 2, . . . , 6 and −1, −2, . . . , −6) for 16 sensors from the samemodule. Note that only one measurement is made for 7 and −7 mA, so theseare not included. The average σ_(RLoop) for the tracks excluding R4 is:0.084Ω² with a standard deviation of 0.068Ω². The damaged track has aσ_(RLoop) of 2.9Ω², which is 41 standard deviations over the average ofthe good tracks.

Other illustrative methods for detecting damage to magnetic sensors aredisclosed in U.S. Patent Pub. No. 2009/0268324A1 entitled “METHODS FORDETECTING DAMAGE TO MAGNETORESISTIVE SENSORS,” which is hereinincorporated by reference. Such methods are useable in variousembodiments of the present invention.

Any method, including those methods presented herein, may be used toverify that the magnetic sensor has been repaired.

Recovery of Pinned Layer Reversal

A previous section showed how one can detect that magnetic state of thesensor using R_(pn) and/or R_(pnIscaled). FIG. 4 b shows howR_(pnIscaled) changes with current magnitude. For the particular cabledsensors studied, currents through R_(s) of 150±25 mA, or V_(cable)7.9±1.3 V can be used to cause a pinned layer reversal. Below about 125mA (6.6 V), pinned layer reversal does not occur for these sensors, butmight be appropriate for other devices. Above about 175 mA (9.3 V), theparticular sensor becomes irreversibly damaged. To recover a sensorwhich has undergone pinned layer reversal, the CDE event is preferablychosen to be above the reversal threshold level (V_(flip)) but below thepermanent damage level (V_(pd)), but with the polarity chosen to alignthe pinned layers in the proper direction. For these sensors, 150±25 mAis a good choice. This is achieved by setting the voltage on the cableto 7.9±1.3 V. As seen in FIG. 2, the current through R_(s) is linear inV_(cc).

GMR sensors are essentially sheet resistors with a thickness, a length(track width, W) and a height (stripe height, H). The exact equationsgoverning the current necessary to cause pinned layer reversal versussensor geometry are complicated but within the skill of one skilled inthe art armed with the teachings herein. The general trend is for therequired current to increase with increasing sensor stripe height. Thestripe height can be determined by measuring the resistance (R_(cold))of the sensor at a low bias current. Preferably, the current issufficiently low as to cause minimal Joule heating of the sensor. A 1 mAbias current is a reasonable choice for measuring R_(cold) for GMRsensors used to measure magnetic densities of the order of 1 GBit/in² orlower since the sensor does not heat appreciably with such a current.For GMR sensors used to measure higher data densities, a lower biascurrent might be chosen, but this is generally understood by thoseskilled in the art. Equation 7 is a simplified equation which relatesthe stripe height to R_(cold):

H=R _(sheet) W/[R _(cold) −R _(lead) −R _(wire)],  7.

In Equation 7, R_(wire) is the resistance of any wires/cables attachedto the sensor, and R_(lead) is any resistance internal to the waferwhich attaches the external wires to the sheet portion of the sensor.R_(sheet) is the sheet resistance of the GMR sensor. The currentrequired to set the pinned layer to the proper level can be determinedon a few parts which span the range of H or W allowed.

The CDE process can also be used to recover parts with diodes attachedto them. FIGS. 9 a and 9 b respectively show R_(pnIscale) versus voltageand current for a CDE event of servos and readers with diode protectionaccording to one exemplary embodiment. Positive voltages were applied todevices R2, R4 and S17 and negative voltages to devices R3 and R5. Thedischarge currents through the sensor are reverse (forward) directionswith positive (negative) voltages on the cable. Pinned layer reversalwas seen for R2 and R4 at currents of ˜600 and 700 mA or voltages forV_(cable) of 30 to 35 V. Thus, recovering a sensor with pinned layerreversal can be achieved in one embodiment by applying a voltage ofbetween −40 to −80V without damaging the sensor. Recovery is thenverified using the R_(pn) and/or R_(pnI) or and/or R_(pnscale) and/orR_(pnIscale) and/or R_(loop) and/or R_(loop) _(—) _(p) and/or R_(loop)_(—) _(n) and/or σ_(RLoop) _(—) _(n) and/or σ_(RLoop) _(—) _(p) and/orσ_(RLoop) as discussed above.

Recovery of a diode protected sensor with multiple domains within thesensor is seen in FIGS. 5 a-b and 6 a-b. The sensor was recovered withthe CDE pulse for cycle 6. The reader had diode protection, so thecurrents required to recover the sensor were very high (1.3 A throughthe 10 ohm shunt resistor in that example). As discussed earlier,recovery was verified using the R_(pn) and/or R_(pnI) or and/orR_(pnscale) and/or R_(pnIscale) and/or R_(loop) and/or R_(loop) _(—)_(p) and/or σ_(RLoop) _(—) _(n) and/or σ_(RLoop) _(—) _(n) and/orσ_(RLoop) _(—) _(p) and/or σ_(RLoop) as discussed above.

It will be clear that the various features of the foregoingmethodologies may be combined in any way, creating a plurality ofcombinations from the descriptions presented above.

It will also be clear to one skilled in the art that the methodology ofthe present invention may suitably be embodied in a logic apparatuscomprising logic to perform various steps of the methodology presentedherein, and that such logic may comprise hardware components or firmwarecomponents.

It will be equally clear to one skilled in the art that the logicarrangement in various approaches may suitably be embodied in a logicapparatus comprising logic to perform various steps of the method, andthat such logic may comprise components such as logic gates in, forexample, a programmable logic array. Such a logic arrangement mayfurther be embodied in enabling means or components for temporarily orpermanently establishing logical structures in such an array using, forexample, a virtual hardware descriptor language, which may be storedusing fixed or transmittable carrier media.

It will be appreciated that the methodology described above may alsosuitably be carried out fully or partially in software running on one ormore processors (not shown), and that the software may be provided as acomputer program element carried on any suitable data carrier (also notshown) such as a magnetic or optical computer disc. The channels for thetransmission of data likewise may include storage media of alldescriptions as well as signal carrying media, such as wired or wirelesssignal media.

Embodiments of the present invention may suitably be embodied as acomputer program product for use with a computer system. Such animplementation may comprise a series of computer readable instructionseither fixed on a tangible medium, such as a computer readable medium,for example, diskette, CD-ROM, ROM, or hard disk, or transmittable to acomputer system, via a modem or other interface device, over either atangible medium, including but not limited to optical or analoguecommunications lines, or intangibly using wireless techniques, includingbut not limited to microwave, infrared or other transmission techniques.The series of computer readable instructions embodies all or part of thefunctionality previously described herein.

Those skilled in the art will appreciate that such computer readableinstructions can be written in a number of programming languages for usewith many computer architectures or operating systems. Further, suchinstructions may be stored using any memory technology, present orfuture, including but not limited to, semiconductor, magnetic, oroptical, or transmitted using any communications technology, present orfuture, including but not limited to optical, infrared, or microwave. Itis contemplated that such a computer program product may be distributedas a removable medium with accompanying printed or electronicdocumentation, for example, shrink-wrapped software, pre-loaded with acomputer system, for example, on a system ROM or fixed disk, ordistributed from a server or electronic bulletin board over a network,for example, the Internet or World Wide Web.

Communications components such as input/output or I/O devices (includingbut not limited to keyboards, displays, pointing devices, etc.) can becoupled to the system either directly or through intervening I/Ocontrollers.

Communications components such as buses, interfaces, network adapters,etc. may also be coupled to the system to enable the data processingsystem, e.g., host, to become coupled to other data processing systemsor remote printers or storage devices through intervening private orpublic networks. Modems, cable modem and Ethernet cards are just a fewof the currently available types of network adapters.

It will be further appreciated that embodiments of the present inventionmay be provided in the form of a service deployed on behalf of acustomer to offer service on demand.

Illustrative Environment

The foregoing teachings apply to any type of magnetic sensor, includingthose for tape head, disk drive heads, and other types of sensors.

FIG. 10 illustrates a simplified tape drive 100 of a tape-based datastorage system, which may be employed in the context of the presentinvention. While one specific implementation of a tape drive is shown inFIG. 10, it should be noted that the embodiments described herein may beimplemented in the context of any type of tape drive system.

As shown, a tape supply cartridge 120 and a take-up reel 121 areprovided to support a tape 122. One or more of the reels may form partof a removable cassette and are not necessarily part of the system 100.The tape drive, such as that illustrated in FIG. 10, may further includedrive motor(s) to drive the tape supply cartridge 120 and the take-upreel 121 to move the tape 122 over a tape head 126 of any type.

Guides 125 guide the tape 122 across the tape head 126. Such tape head126 is in turn coupled to a controller assembly 128 via a cable 130. Thecontroller 128 typically controls head functions such as servofollowing, writing, reading, etc. The cable 130 may include read/writecircuits to transmit data to the head 126 to be recorded on the tape 122and to receive data read by the head 126 from the tape 122. An actuator132 controls position of the head 126 relative to the tape 122.

An interface 134 may also be provided for communication between the tapedrive and a host (integral or external) to send and receive the data andfor controlling the operation of the tape drive and communicating thestatus of the tape drive to the host, all as will be understood by thoseof skill in the art.

By way of example, FIG. 11 illustrates a side view of a flat-lapped,bi-directional, two-module magnetic tape head 200 which may beimplemented in the context of the present invention. As shown, the headincludes a pair of bases 202, each equipped with a module 204, and fixedat a small angle α with respect to each other. The bases are typically“U-beams” that are adhesively coupled together. Each module 204 includesa substrate 204A and a closure 204B with a thin film portion, commonlyreferred to as a “gap” in which the readers and/or writers 206 areformed. In use, a tape 208 is moved over the modules 204 along a media(tape) bearing surface 209 in the manner shown for reading and writingdata on the tape 208 using the readers and writers. The wrap angle θ ofthe tape 208 at edges going onto and exiting the flat media supportsurfaces 209 are usually between ⅛ degree and 4½ degrees.

The substrates 204A are typically constructed of a wear resistantmaterial, such as a ceramic. The closures 204B made of the same orsimilar ceramic as the substrates 204A.

The readers and writers may be arranged in a piggyback configuration.The readers and writers may also be arranged in an interleavedconfiguration. Alternatively, each array of channels may be readers orwriters only. Any of these arrays may contain one or more servo readers.

While various embodiments have been described above, it should beunderstood that they have been presented by way of example only, and notlimitation. Thus, the breadth and scope of an embodiment of the presentinvention should not be limited by any of the above-described exemplaryembodiments, but should be defined only in accordance with the followingclaims and their equivalents.

1. A system, comprising: a power supply for charging a lead of amagnetic sensor to a voltage; an interface for operatively coupling thepower supply to the lead of the magnetic sensor; a relay for selectivelycoupling the lead of the magnetic sensor to ground for causing adischarge event, wherein the discharge event reverses a magneticorientation of a pinned layer of the magnetic sensor; and a shortingresistor between the relay and ground.
 2. A system as recited in claim1, further comprising an inductive choke, wherein leads from the powersupply are coupled to the inductive choke thereby creating a highimpedance path for unwanted currents.
 3. A system as recited in claim 1,wherein a pulse width of the discharge event has a duration of less thanabout 45 nanoseconds (ns).
 4. A system as recited in claim 1, whereinthe interlace is adapted for coupling to a cable positioned between theinterface and the magnetic sensor, wherein the cable functions asextensions of the lead and a second lead of the magnetic sensor.
 5. Asystem as recited in claim 1, further comprising a capacitor coupledbetween another lead of the magnetic sensor and ground.
 6. A system asrecited in claim 1, further comprising a mechanism to measure aresistance of the sensor over multiple bias currents.
 7. A system asrecited in claim 6, further comprising a relay for selectively couplingthe lead of the magnetic sensor to the mechanism, and another relay forselectively coupling a second lead of the magnetic sensor to themechanism, wherein the mechanism supplies the multiple bias currents. 8.A system as recited in claim 6, further comprising logic for determiningwhether the magnetic sensor is damaged based on the measured resistancesof the magnetic sensor at the bias currents using at least one of thefollowing equations:R _(pn)(I _(bias))=[R(+I _(bias))−R(−I _(bias))];R _(pnscaled)(I _(bias))=[R(+I _(bias))−R(−I _(bias))]/[[R(+I_(bias))−R(−I _(bias))]];R _(pnI)(I _(bias))=[R(+I _(bias))−R(−I _(bias))]/I _(bias);R _(pnIscaled)(I _(bias))=[R(+I _(bias))−R(−I _(bias))]/[I _(bias)*[R(+I _(bias))−R(−I _(bias))]]; <f>≡Σ_({Ibias}) f(I_(bias)), wheref(I_(bias)) is selected from a group consisting of: R_(pn)(I_(bias)),R_(pnI)(I_(bias)),R_(pnscale)(I_(bias)) and R_(pnscale)(I_(bias));R_(loop) _(—) _(p)=Σ_({Ibias})|R_(up)(I_(bias))−R_(down)(I_(bias))|;R_(loop) _(—) _(n)=Σ_({Ibias})|R_(up)(−I_(bias))−R_(down)(−I_(bias))|;R_(loop)=R_(loop) _(—) _(p)+R_(loop) _(—) _(n);σ_(RLoop) _(—) _(p)=Σ_({Ibias})[R_(up)(−I_(bias))−R_(down)(−I_(bias))]²;σ_(RLoop) _(—) _(n)=Σ_({Ibias})[R_(up)(−I_(bias))−R_(down)(−I_(bias))]²,andσ_(RLoop)=σ_(RLoop) _(—) _(p)+R_(loop) _(—) _(n); and where (I_(bias))is one or more predetermined bias currents.
 9. A system as recited inclaim 1, with the proviso that the system does not include a groundplane.
 10. A system for testing a magnetic sensor, comprising: adischarge circuit to cause a discharge event on a magnetic sensor; abias generation circuit to apply at least one first bias current to thesensor and at least one second bias current to the sensor, the secondbias current being different than the first bias current; a resistancedetermination circuit to determine a resistance of the magnetic sensorat each of the applied bias currents; and a damage determination circuitto determine whether the magnetic sensor is damaged and/or was fixed bya discharge event based on the resistances.
 11. A system as recited inclaim 10, wherein the discharge circuit includes a shorting resistorconnected between one lead of the magnetic sensor and ground whereby aconnection between the one lead and the shorting resistor coupled toground is made or broken using a relay.
 12. A system as recited in claim10, wherein a pulse width of the discharge event has a duration of lessthan about 45 nanoseconds (ns).
 13. A system as recited in claim 10,wherein a cable is coupled between the logic for causing the dischargeevent and the magnetic sensor, wherein the cable functions as extensionsof a leads of the magnetic sensor.
 14. A system as recited in claim 10,wherein the at least one second bias current is of an opposite polarityas the first bias current.
 15. A system as recited in claim 10, whereinthe damage determination circuit uses at least one of the followingequations:R _(pn)(I _(bias))=[R(+I _(bias))−R(−I _(bias))];R _(pnscaled)(I _(bias))=[R(+I _(bias))−R(−I _(bias))]/[[R(+I_(bias))−R(−I _(bias))]];R _(pnI)(I _(bias))=[R(+I _(bias))−R(−I _(bias))]/I _(bias);R _(pnIscaled)(I _(bias))=[R(+I _(bias))−R(−I _(bias))]/[I _(bias)*[R(+I _(bias))−R(−I _(bias))]]; <f>≡Σ_({Ibias}) f(I_(bias)), wheref(I_(bias)) is selected from a group consisting of: R_(pn)(I_(bias)),R_(pnI)(I_(bias)),R_(pnscale)(I_(bias)) and R_(pnscale)(I_(bias));R_(loop) _(—) _(p)=Σ_({Ibias})|R_(up)(I_(bias))−R_(down)(I_(bias))|;R_(loop) _(—) _(n)=Σ_({Ibias})|R_(up)(−I_(bias))−R_(down)(−I_(bias))|;R_(loop)=R_(loop) _(—) _(p)+R_(loop) _(—) _(n);σ_(RLoop) _(—) _(p)=Σ_({Ibias})[R_(up)(−I_(bias))−R_(down)(−I_(bias))]²;σ_(RLoop) _(—) _(n)=Σ_({Ibias})[R_(up)(−I_(bias))−R_(down)(−I_(bias))]²,andσ_(RLoop)=σ_(RLoop) _(—) _(p)+R_(loop) _(—) _(n); and where (I_(bias))is one or more predetermined bias currents.
 16. A method, comprising:measuring resistances of a magnetic sensor at multiple bias currents;determining that the magnetic sensor is damaged based on the measuredresistances; selecting a bias voltage sufficient to cause a dischargeevent that repairs the damaged magnetic sensor to a proper magneticstate thereof; applying the bias voltage to the magnetic sensor; andcoupling the lead of the magnetic sensor to ground after applying thebias voltage for causing the discharge event, wherein the dischargeevent fixes the damaged magnetic sensor.
 17. A method as recited inclaim 16, further comprising determining a resistance (Rcold) of thesensor using a current sufficiently low as to cause minimal Jouleheating of the sensor, and determining a voltage level (Vflip)sufficient to cause pinned layer reversal and a voltage level (Vpd)sufficient to cause permanent damage to the magnetic sensor, wherein theselected bias voltage is between the voltage level (Vflip) sufficient tocause pinned layer reversal and the voltage level (Vpd) sufficient tocause permanent damage to the magnetic sensor.
 18. A method as recitedin claim 16, wherein a pulse width of the discharge event has a durationof less than about 45 nanoseconds (ns).
 19. A method as recited in claim16, wherein the step of determining whether the magnetic sensor isdamaged uses at least one of the following equations:R _(pn)(I _(bias))=[R(+I _(bias))−R(−I _(bias))]:R _(pnscaled)(I _(bias))=[R(+I _(bias))−R(−I _(bias))]/[[R(+I_(bias))−R(−I _(bias))]];R _(pnI)(I _(bias))=[R(+I _(bias))−R(−I _(bias))]/I _(bias);R _(pnIscaled)(I _(bias))=[R(+I _(bias))−R(−I _(bias))]/[I _(bias)*[R(+I _(bias))−R(−I _(bias))]]; <f>≡Σ_({Ibias}) f(I_(bias)), wheref(I_(bias)) is selected from a group consisting of: R_(pn)(I_(bias)),R_(pnI)(I_(bias)),R_(pnscale)(I_(bias)) and R_(pnIscale)(I_(bias));R_(loop) _(—) _(p)=Σ_({Ibias})|R_(up)(I_(bias))−R_(down)(I_(bias))|;R_(loop) _(—) _(n)=Σ_({Ibias})|R_(up)(−I_(bias))−R_(down)(−I_(bias))|;R_(loop)=R_(loop) _(—) _(p)+R_(loop) _(—) _(n);σ_(RLoop) _(—) _(p)=Σ_({Ibias})[R_(up)(I_(bias))−R_(down)(I_(bias))]²;σ_(RLoop) _(—) _(n)=Σ_({Ibias})[R_(up)(−I_(bias))−R_(down)(−I_(bias))]²,and and where (I_(bias)) is one or more predetermined bias currents. 20.A computer program product for determining whether a magnetic sensor isdamaged, the computer program product comprising: a computer readablestorage medium having computer readable program code embodied therewith,the computer readable program code comprising: computer readable programcode configured to receive measured resistances of a magnetic sensor atmultiple bias currents computer readable program code configured todetermine that the magnetic sensor is damaged based on the measuredresistances using at least one of the following equations:R _(pn)(I _(bias))=[R(+I _(bias))−R(−I _(bias))];R _(pnscaled)(I _(bias))=[R(+I _(bias))−R(−I _(bias))]/[[R(+I_(bias))−R(−I _(bias))]];R _(pnI)(I _(bias))=[R(+I _(bias))−R(−I _(bias))]/I _(bias);R _(pnIscaled)(I _(bias))=[R(+I _(bias))−R(−I _(bias))]/[I _(bias)*[R(+I _(bias))−R(−I _(bias))]]; <f>≡Σ_({Ibias}) f(I_(bias)), wheref(I_(bias)) is selected from a group consisting of: R_(pn)(I_(bias)),R_(pnI)(I_(bias)),R_(pnscale)(I_(bias)) and R_(pnIscale)(I_(bias));R_(loop) _(—) _(p)=Σ_({Ibias})|R_(up)(I_(bias))−R_(down)(I_(bias))|;R_(loop) _(—) _(n)=Σ_({Ibias})|R_(up)(−I_(bias))−R_(down)(−I_(bias))|;R_(loop)=R_(loop) _(—) _(p)+R_(loop) _(—) _(n)σ_(RLoop) _(—) _(p)=Σ_({Ibias})[R_(up)(I_(bias))−R_(down)(I_(bias))]²;σ_(RLoop) _(—) _(n)=Σ_({Ibias})[R_(up)(−I_(bias))−R_(down)(−I_(bias))]²,andσ_(RLoop)=σ_(RLoop) _(—) _(p)+σ_(RLoop) _(—) _(n); and where (I_(bias))is one or more predetermined bias currents.